The United States confirmed on Wednesday that the video purporting to show US journalist Steven Sotloff being executed by Takfiri group ISIL in Iraq is authentic, with President Barack Obama vowed “not to be intimidated”.

The United States confirmed on Wednesday that the video purporting to show US journalist Steven Sotloff being executed by Takfiri group ISIL in Iraq is authentic, with President Barack Obama vowed “not to be intimidated”.

"The US Intelligence Community has analyzed the recently released video showing US citizen Steven Sotloff and has reached the judgment that it is authentic," said National Security Council spokesperson Caitlin Hayden.

In the footage release by ISIL, Sotloff calmly addresses the camera to say he is a victim of President Barack Obama's decision to press on with air strikes in Iraq against the Takfiris. The video later shows a black-clad militant cutting his throat, before displaying his severed head.

Last month another American reporter, James Foley, was murdered in a similar fashion.

Obama vowed Wednesday that the United States would not be intimidated by the beheading but acknowledged the fight against the Takfiri militants would take time.

Obama pledged that justice would be done to the ISIL killers of 31-year-old reporter Steven Sotloff, wherever they hid and however long it took.
But he warned that eliminating the threat posed to the region by the group from its bases in Iraq and Syria would take time.

Obama said the whole world had been repulsed by the barbarism of Sotloff's murder but "we will not be intimidated".

"Those who make the mistake of harming Americans will learn that we will not forget and that our reach is long and that justice will be served," he said.

The US president said Washington was determined to halt the ISIL threat to the region but warned it would depend on close cooperation with partners in the region.

In the video posted on Tuesday, ISIL also warned a British hostage would be next unless London backs off from its support for Washington's air campaign.

For his part, British Prime Minister David Cameron said the video depicted an "absolutely disgusting, despicable act" and convened a meeting of security chiefs to discuss how to tackle the ISIL threat.

The masked executioner in the video spoke with a London accent and claimed to be the same man, confirmed by UK security services as a Briton, who beheaded Foley.

"I'm back, Obama, and I'm back because of your arrogant foreign policy towards the Islamic State," the black-clad Takfiri militant says, wielding a combat knife.

"So just as your missiles continue to strike the necks of our people, our knife will continue to strike the necks of your people," he declares, before reaching round to cut his captive's throat.